Since a photographic process using silver halide is excellent in photographic characteristics such as sensitivity, gradation control, etc., as compared to other photographic processes such as an electrophotogrpahic process and a diazo photographic process, the silver halide photographic process has been most widely used. Recently, however, a technique capable of easily and quickly obtaining images has been developed by performing the image formation of a light-sensitive material using silver halide by a dry treatment such as heating in place of performing the image-formation by a conventional wet treatment such as a treatment by a developing liquid, etc.
A heat developable light-sensitive material is known in the field of the art and such a heat developable light-sensitive material and a process of processing it are described in, for example, Shashin Kogaku no Kiso (The Basis of Photographic Engineering), pages 553-555, published by Korona K. K. in 1979; EIZO JOHO (Image Information), page 40, published in Apr. 1978; Nebletts Handbook of Photography and Reprography, 7th Ed., pages 32-33, published by Van Nostrand Reinhold Company; U.S. Pat. Nos. 3,152,904; 3,301,678; 3,392,020 and 3,457,075; U.K. Patent Nos. 1,131,108 and 1,167,777; and Research Disclosure, pages 9-15 Jun. 1978 (RD-17029).
Various processes of obtaining color images by a dry system have been proposed. For example, for a process of forming color images by the combination of the oxidation product of a developing agent and couplers, a reducing agent such as p-phenylenediamines and phenolic or active methylene couplers are disclosed in U.S. Pat. No. 3,531,286, p-aminophenolic reducing agents are disclosed in U.S. Pat. No. 3,761,270, sulfonamidophenolic reducing agents are disclosed in Belgian Patent No. 802,519 and Research Disclosure, pages 31-32, Sept. 1975, and the combination of the sulfonamidophenolic reducing agents and four-equilvalent couplers is disclosed in U.S. Pat. No. 4,021,240.
However, the foregoing processes have a fault that color images formed become turbid since an image of reduced image and a color image are simultaneously formed at the exposed area of a light-sensitive material after heat development. For overcoming the fault, there is a method of removing the silver image by a liquid treatment or a method of transferring the dye only to other layer such as a sheet having an image-receiving layer. However, such a method has also a fault that it is not easy to transfer the dye only separately from unreacted materials.
Also, a method of introducing a nitrogen-containing heterocyclic group in a dye, forming a silver salt, and liberating the dye by heat development is described in Research Disclosure, pages 54-58, May 1978 (RD-16966). However, the method cannot form clear images since it is difficult to liberate the dye at unexposed areas and hence is not suitable for general use.
Furthermore, useful dyes and bleaching processes for forming positive dye images by a heat-sensitive silver dye bleach process are described in, for example, Research Disclosure, pages 30-32, Apr. 1976 (RD-14433); ibid, pages 14-15, Dec. 1976 (RD-15227); U.S. Pat. No. 4,235,957, etc. However, these methods have faults that additional material and step, such as an activator sheet for accelerating the bleach of the dye and a step of superposing the activator sheet and heating are required and also the color images formed by the methods cannot be stably preserved for a long period of time since the color images are gradually bleached by reduction with free silver, etc., which are co-existing.
Also, a method of forming color images utilizing leuco dyes is described in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617. However, the method has also a fault that it is difficult to stably incorporate a leuco dye in a photographic material and hence the photographic material is gradually colored during the preservation thereof.
The inventors previously proposed a novel light-sensitive material capable of overcoming the foregoing faults in the conventional methods and an image-forming process for the light-sensitive material (Japanese Patent Application No. 157,798/'81). In the foregoing image-forming process, a light-sensitive material capable of releasing a mobile hydrophilic dye by a simple method of heating in a substantially water free state is heated to release a mobile dye and the mobile dye is transferred into a dye-fixing layer mainly in the presence of a solvent.